A conventional rudder system for very large vessels is such that, as shown in FIG. 21-FIG. 22, a rudder 51, an overwhelming majority of which is of so-called Mariner type, is disposed behind a propeller 3. The rudder 51 is supported free rotatably by a pintle 54 provided at a lower end of a streamlined horn 53, which is protruded downward from a bottom center of a stem 52. The maximum rotatable angle of the rudder 51 is 35° at its one end and 35° at its other end, 70° in total.
Conventionally, a rudder area has been determined based on actual results so that a value that a projected flood area, namely a multiplier of ship length and draft, is divided by a rudder area (rudder area ratio) is within a certain range, though different depending on ship length and ship kind.
Recently, however, maneuverability of very large vessels such as a very large crude oil carrier etc., which embrace problems in course stability and follow-up controllability, when navigating in narrow waters and ports, has come to the front, and it is the existing state that, in order to meet the IMO (International Maritime Organization) maneuverability criteria, such a measure is taken as to not only alter ship form but also reduce rudder area ratio, namely increase rudder area. Accordingly, it is the present state that globally very large vessels are provided with such a large rudder 51 that its average chord length c′ extends to such degree as 110% of a propeller diameter d.
Besides, such a concept is in existence as to provide two propellers, and a rudder is provided behind the respective propeller. This simply arranges two sets of the above-mentioned system of a single propeller and a single rudder, aiming at safety when either of the propulsion engines fails. In this case, it is so arranged that two rudders are turned synchronously up to the maximum rudder angle of 35° port and 35° starboard.
The necessity for increasing rudder area in the conventional rudder system as mentioned above has caused problems such that not only the rudder becomes heavy in construction and requires large steering gear capacity, but also it may bring a lowering of propulsive performance, and that, as the case may be, there is possibility of requiring enlargement of hull dimension to secure space for the enlarged rudder, all these causing economic loss.
In addition, there has been a problem that, even if rudder area is increased, an increase of the rudder force is not so high and it is not so effective for improvement of maneuverability due to low speed when navigating in narrow waters and ports, despite the need for high maneuverability in the very narrow waters and ports.
Furthermore, in a conventional rudder, an increase of rudder operating angle has been less effective for improvement of maneuverability because lift of the rudder suddenly decreases when exceeding 35°.
Furthermore, there has been a problem that the conventional rudder system as mentioned above may make the ship incapable of maneuvering and may cause lost safety in case either rudder or steering gear fails. If two sets of the conventional rudder system are provided, such a problem is solved, but it would be impractical because it causes another problem that propulsive efficiency is lowered and cost becomes high due to enlarged space requirement and facilities. In addition, there is a problem that there is a case where rudder force can not be effectively generated at large rudder angles by interfering action of a stream that flows between two rudders as they are turned synchronously.
As for rudder angle control system for a ship provided with twin rudders, a conventional system has been such that, as shown in FIG. 23 for example, an auto-pilot 62 controls a port rudder 61p and a starboard rudder 61s so that they are turned synchronously, and that the respective rudder can be turned up to the same maximum rudder angle toward port side and starboard side.
Namely, when a rudder angle order signal δi is issued from either an automatic steering apparatus 62a or a steering wheel 62b of the auto-pilot 62, the signal δi is input into a port control amplifier 63p, as it is, for controlling a port rudder 61p and a starboard control amplifier 63s for controlling a starboard rudder 61s, respectively, in a synchronous manner. Hereby the port and starboard control amplifiers 63p, 63s issue order signals to a port hydraulic pump unit 65p of a port steering gear 64p so as to make a port rudder 61p operate, and a starboard hydraulic pump unit 65s of a starboard steering gear 64s so as to make a starboard rudder 61s operate, respectively, and the port and starboard steering gears 64p, 64s and the port and starboard rudders 61p, 61s begin to turn synchronously in the same direction.
A moving amount of the port rudder 61p is fed back to the port control amplifier 63p as a port rudder angle feedback signal δfp, and a moving amount of the starboard rudder 61s is fed back to the starboard control amplifier 63s as a starboard rudder angle feedback signal δfs, respectively. When the signals come to such relation as δfp=δi and δfs=δi, the control amplifiers 63p, 63s make operation of the port and starboard hydraulic pump units 65p, 65s stop, respectively, and the port and starboard rudders 61p, 61s are kept at the rudder angle δi ordered by the automatic steering apparatus 62a or the steering wheel 62b of the auto-pilot 62.
According to the conventional auto-pilot as abovementioned, there is such a problem that two rudders are unable to effectively generate rudder force at large rudder angles due to synchronous operation of two rudders, which causes mutual interfering action of a deflected propeller slip stream that streams between the port and starboard rudders.
In addition, a rudder's working angle range becomes necessarily large because the maximum inboard operable angle is equal to the maximum outboard operable angle, and thus there is such a problem that the maximum operable angle should be compelled to be restricted due to a restriction on steering gear mechanism, resulting in incapability of developing large rudder force.
Furthermore, the conventional auto-pilot does not manage such control as to, in a twin rudder arrangement, turn the respective rudders toward outboard and give a ship brake force against onward movement, while such a special character of control can be utilized for crash stopping (or crash astern) maneuver of a ship.
In case of crash stopping (or crash astern) maneuver of a ship, it is performed by means of reversing propeller revolution by reversing operation of a main engine or a clutch provided in a reduction gear to stop an onward moving ship and further make the ship go astern.
On this occasion, the ship continues moving onward by large inertia force even after fuel supply to a main engine is stopped, and a propeller idles. If the propeller is so controlled as to be reversed on this condition, the propulsive system will come to be over-loaded; accordingly, it is usual practice that reversing the main engine or the clutch of reduction gear is carried out after onward moving speed of the ship by inertia force or free rotating speed of the propeller has come down to a certain value in the course of nature.
There is a problem in that a long time is required until it becomes possible to give the ship positive astern power, and in the meanwhile, the ship continues running onward by inertia force, covering an extremely long distance, which means that risk of collision increases, and that ship maneuvering crew is compelled to accept the great labors for avoiding risk.
Furthermore, in case that a ship is propelled by a main diesel engine and a propeller is of fixed pitch, there is a problem that, as the main diesel engine revolution is unable to be decreased lower than “dead slow” which is the lowest allowable revolution, a considerably high undesirable ship speed remains. In case a twin rudder arrangement is equipped, however, it is possible that, by such means that the respective rudders are turned toward outboard and their turned angles are controlled, a ship speed can be decreased beyond the speed corresponding to the main diesel engine dead slow revolution voluntarily, within the limit of the rudders' maximum operable angles toward outboard, and that ship's heading can be controlled. Nevertheless, the conventional auto-pilot does not manage such control.
The present invention aims at offering such a twin-rudder system for very large vessels that two high lift rudders, respective blade chord length of which is made about a half of a propeller diameter, are arranged behind a single propeller, and that respective rudder angles are controlled so that they can co-work most effectively, which enables:    To provide a very large vessel with excellent maneuverability, including braking ability, not only at high speed navigation, but also especially at low speed navigation in narrow waters and ports;    Nevertheless, to secure propulsive performance equal with or higher than that of the case that a conventional rudder system is equipped;    To make rudder construction light;    To shorten ship length or increase stowage capacity due to shortened rudder sizes;    To reduce required capacity and required working angle for steering gears;    To adopt a simple rudder supporting system of hanging type;    To secure ship maneuvering ability with safety, even in case that something has been wrong with either of the rudders or the steering gear;    To make two rudders effectively generate rudder force, even in case the rudders are largely steered when a ship is at turning maneuver or head changing maneuver, by such a means that two rudders are less influenced by mutual interfering action of a deflected propeller slip stream streaming between two rudders;    To decrease a required working angle range of steering gears in spite of the increased maximum rudder operable angle;    To greatly shorten a ship's crash stopping distance, when the ship is at crash stopping (or crash astern) maneuver, by utilizing two rudders as brake against onward movement of the ship; and    To reduce a ship speed beyond the speed corresponding to the lowest allowable revolution of a main diesel engine, with an ability of controlling ship's heading, by utilizing two rudders.